Spatial Variability of Soil Aggregate Size Fractions Driven by Precipitation and Nitrogen Fertilizer at the Regional Scale in Southwestern China

Detailed information on the spatial variability of soil aggregate-size fractions (SASF) is crucial for soil erosion modelling and agricultural production. The effects of intrinsic and extrinsic factors on SASF have been widely studied at the grain to small-watershed scales, but rarely studied at the regional scale. This study aimed to investigate the influence of 19 environmental factors on the spatial variability of SASF in southwestern China, where similar tillage practices were used in local tobacco fields. A total of 2238 soil samples were randomly collected from the topsoil (0–20 cm) for analysis. The random forest model was used to identify the relationship between SASF and environmental factors. Random Forest explained 43–54% of SASF variability. Total precipitation during the non-growing period (NGP) was the main factor influencing the variation of SASF, which was 2 to 3 times more important than total precipitation during the growing season (GP) and nitrogen fertilizer application, which ranked second or third, respectively. After NGP exceeded the threshold values, aggregate formation slowed down, while after GP exceeded the threshold values, aggregate fragmentation accelerated. Additionally, excessive nitrogen fertilization not only negatively affected soil aggregate formation, but also weakened the promotional effects of NGP. Overall, our regional-scale study identified the effects of precipitation and nitrogen fertilization on SASF, which might be useful for regional soil erosion modelling and climate-adapted agricultural policies.